A semiconductor laser consisting of III-V nitride semiconductor material (AlxGayIn1-x-yN (herein, 0≦x≦1, and 0≦y≦1)) such as gallium nitride is a key device for realizing ultra-high density recording on an optical disc and is a blue violet semiconductor laser which is at a level closest to a practical use. Increasing an output of such a blue violet semiconductor laser allows not only rapid writing on optical discs but also application to laser displays. This is an essential technology for developing a new technical field.
In optical disc systems of a recording and reproduction type, a semiconductor laser with a high output is desirable. As an effective way to increase an output, a method to render reflectances of end surfaces of a resonator asymmetrical is known (see, for example, “Handotai reza (semiconductor laser)”; Kenichi Iga Ed.; First edition; Ohmsha Ltd.; Oct. 25, 1994; p. 238). It is a general method for semiconductor lasers used for writing on optical discs. In this method, end surfaces forming a resonator are coated with dielectric films so that the end surfaces have asymmetrical reflectances. Among the end surfaces forming the resonator, a front end surface from which a laser light is emitted has a low reflectance, and a rear end surface on the opposite side has a high reflectance (for example, 10% for the front end surface and 90% for the rear end surface). A reflectance of a dielectric multilayer film can be controlled based on a refractive index, a film thickness and the number of layers laminated of a used dielectric member.
A semiconductor laser is mounted (assembled) to a can package as shown in FIG. 11. The package consists of a base 803 and a cap 804. A semiconductor laser 801 and a submount 802 which is a heat radiator for the semiconductor laser 801 are mounted to the base 803. A space inside the cap is sealed with nitrogen (N2) gas or the like.
The cap consists of glass 806 for taking out light and a metal base (can 805). The cap is adhered with a fusible glass 807 (can be fixed at several hundreds degree) in order to keep air tightness.
Such a semiconductor laser suffers from a problem in that dust and mold may adhere to a surface of the laser package and affect an output property. In order to solve this problem, a method of forming a film having a photocatalytic function on a surface of a package of a laser has been proposed (see Japanese Laid-Open Publication No. 2003-59087).
A solid-state laser has also been developed as a coherent light source with a high output. A wavelength conversion element is inserted into a resonator of the solid-state laser. This enables generating visible light with a high output. In such a solid-state laser, dust attached to laser end surfaces causes deterioration of an outputting property and shortens the life of the laser. In order to solve this problem, a method of providing a film having a photocatalytic effect on a surface of an optical component has been proposed (see Japanese Laid-Open Publication No. 2001-70787). A structure in which a film having a photocatalytic function is formed on a solid-state laser resonator mirror is also shown.
In conventional light sources, it is intended that light from the coherent light source is used to activate a catalytic function of a photocatalyst. However, photocatalyst depends on wavelengths. For activating the photocatalytic functions, wavelengths of the coherent light sources which can be used are limited. Specifically, for activating a photocatalyst efficiently, light sources with a wavelength of 390 nm or shorter are required. Thus, there is a problem in that coherent light sources which can utilize a photocatalyst are limited to light sources with short wavelengths of 390 nm or shorter.
An object of the present invention is to solve the above described conventional problem and provide a coherent light source with eased limitations on wavelengths at emission. Another object of the present invention is to provide an optical system with eased limitations on wavelengths at emission.